Printing plates and a method for their preparation

ABSTRACT

A method for the preparation of a lithographic printing plate comprises forming an oleophilic image on the surface of a hydrophilic support by depositing, preferably by ink-jetting, the image on the surface using an aqueous dispersion of an oligomer having in the molecule both hydrophilic and hydrophobic groups. The number of repeating units in the oligomer may be from 2 to 10 and the number of hydrophilic groups in the oligomer may also be from 2 to 10. Preferably the molecular weight of the oligomer is from about 500 to about 5000.

FIELD OF THE INVENTION

This invention relates to novel printing plates, to a method for theirpreparation and to a lithographic printing process employing the plates.

BACKGROUND OF THE INVENTION

Printing plates suitable for offset lithographic printing are knownwhich comprise a support having non-image areas which are hydrophilicand image areas which are hydrophobic and ink-receptive.

The art of lithographic printing is based upon the immiscibility of oiland water, wherein the oily material or ink is preferentially retainedby the image area and water or fountain solution is preferentiallyretained by the non-image area. When a suitably prepared surface ismoistened with water and an ink is then applied the background ornon-image area retains the water and repels the ink while the image areaaccepts the ink and repels the water. The ink on the image area is thentransferred to the surface of a material upon which the image is to bereproduced, such as paper, cloth and the like. Commonly the ink istransferred to an intermediate material called the blanket which in turntransfers the ink to the surface of the material upon which the image isto be reproduced.

Ink-jetting is the non-impact method for producing images by thedeposition of ink droplets on a substrate in response to digitalsignals. JP-A-53015905 describes the preparation of a printing plate byink-jetting an alcohol-soluble resin in an organic solvent onto analuminum printing plate. JP-A-56105960 describes the formation of aprinting plate by ink-jetting onto a support e.g. an anodised aluminumplate an ink capable of forming an oleophilic image and containing ahardening substance such as epoxy-soybean oil together with benzoylperoxide or a photo-hardening substance such as an unsaturatedpolyester.

European Patent Application No. 882584 describes a method of preparing aprinting plate comprising producing an oleophilic image on the surfaceof a support by ink-jet printing the image on the surface using anaqueous solution or of a salt of a hydrophobic organic acid e.g. oleicacid. G.B. Patent Application No. 2,332,646 describes a method ofpreparing a printing plate comprising producing an oleophilic image onthe surface of a support by ink-jet printing the image on the surfaceusing an aqueous solution or aqueous colloidal dispersion of a polymerbearing water-solubilising groups wherein the water-solubilising groupsinteract with the surface of the support thereby binding the polymer tothe support and rendering the polymer insoluble.

Problem to be Solved by the Invention

The prior art methods involve the use of organic solvents orphoto-hardenable compounds or polymers which introduces some risk thatthe inlets may become blocked by the polymer.

A solution to these problems has now been invented in which an oligomerhaving hydrophilic and hydrophobic groups in the molecule, is used as anaqueous dispersion to prepare the image on the printing plate.

SUMMARY OF THE INVENTION

According to the present invention a method for the preparation of alithographic printing plate comprises forming an oleophilic image on thesurface of a hydrophilic support by depositing, preferably byink-jetting, the image on the surface using an aqueous dispersion of anoligomer having in the molecule both hydrophilic and hydrophobic groups.

Advantageous Effect of the Invention

The method of the invention offers a rapid, simple and direct way tomake a printing plate from digital data using relatively low costequipment and without light sensitive materials.

Compared with the methods disclosed in the prior art, the method of theinvention requires no processing of the plate and uses dilute aqueousdispersions having a low level of environmental impact and low healthrisk. In addition the risk of the jets being blocked by polymer isreduced.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawing is a sketch showing the formation of a printing plate byink-jetting.

DETAILED DESCRIPTION OF THE INVENTION

The oligomer may be applied as an aqueous solution or an aqueousemulsion.

The term aqueous is intended to include the optional presence of organicliquids such as those that are miscible with water e.g. polyhydricalcohols e.g. ethylene glycol, diethylene glycol, trimethylene glycol ortrimethylol propane.

The proportion of water in the aqueous liquid in which the oligomer isdissolved or dispersed is at least 40%, preferably at least 75%, morepreferably at least 80% by weight.

The oligomer preferably comprises from 2 to 10 repeating units morepreferably 3 to 5 and preferably the number of hydrophilic groups in theoligomer is also from 2 to 10.

Because the oligomer contains both hydrophobic and hydrophilic it willhave the characteristics of a surfactant.

The hydrophilic groups, which may be anionic, serve to bind the oligomerto the hydrophilic surface thereby rendering the oligomer insoluble.

The hydrophobic group may comprise a non polar hydrocarbon portion ofthe molecule.

The hydrophilic groups may be acid groups such as carboxylic, sulphonic,sulphate, phosphate or phosphonic acids. Some or all of such acid groupsmay exist as salts for example those of an alkali metal or ammonium.

The molecular weight of the oligomers is typically in the range fromabout 500 to about 5000, preferably from about 1000 to about 3000.

The support may be any support suitable for printing plates. Typicalsupports include metallic and polymeric sheets or foils. The surface ofthe support may be treated or coated to provide the necessaryinteraction with the oligomer. Examples of surface coatings include ametallic oxide and gelatin coatings.

Preferably a support having a metallic surface is used. Preferably themetallic surface is oxidized. In a particularly preferred embodiment ofthe invention a support having an anodized aluminum surface is employed.

Jet velocity, separation length of the droplets, drop size and streamstability are greatly affected by the surface tension and the viscosityof the aqueous composition. Ink-jet inks suitable for use with ink-jetprinting systems may have a surface tension in the range from 20 to 60,preferably 30 to 50 dynes/cm. Control of the surface tension in aqueousinks may be accomplished by addition of small amounts of surfactants.The level of surfactants to be used can be determined through simpletrial and error experiments. Anionic and non-ionic surfactants may beselected from those disclosed in U.S. Pat. Nos. 5,324,349; 4,156,616;and 5,279,654 as well as many other surfactants known in the ink-jetart. Commercial surfactants include the SURFYNOL brand products (TradeMark) from Air Products; the ZONYL brand products (Trade Mark) rangefrom DuPont; the FLUORAD brand products (Trade Mark) range from 3M andthe AEROSOL brand products (Trade Mark) from Cyanamid. The viscosity ofthe ink is preferably no greater than 20 centipoise e.g. from 1 to 10,preferably from 1 to 5 centipoise at 20° C.

The emulsion used in the ink-jet printer may comprise other ingredients,for example water-soluble liquids or solids with a substantially higherboiling point than water, e.g. ethanediol, as well as other types ofoleophilic precursors such as the sodium salt of oleic acid. A humectantor co-solvent may be included to help prevent the ink from drying out orcrusting in the orifices of the print head. A penetrant may alsooptionally be included to help the ink penetrate the surface of thesupport. A biocide, such as PROXEL (Trade Mark) GXL biocide from ZenecaColours may be added to prevent microbial growth which may otherwiseoccur in the ink over time.

The aqueous emulsion is employed in ink-jet printing wherein drops ofthe emulsion are applied in a controlled fashion to the surface of thesupport by ejecting droplets from a plurality of nozzles or orifices ina print head of an ink-jet printer. Commercially available ink-jetprinters use several different schemes to control the deposition of theink droplets. Such schemes are generally of two types: continuous streamor drop-on-demand. In drop-on-demand systems a droplet of ink is ejectedfrom an orifice directly to a position on the ink receptive layer bypressure created by, for example, a piezoelectric device, an acousticdevice, or a thermal process controlled in accordance with digitalsignals. An ink droplet is not generated and ejected through the orificeof the print head unless it is needed. Ink-jet printing methods andrelated printers are commercially available and need not be described indetail.

The aqueous emulsion may have properties compatible with a wide range ofejecting conditions, e.g. driving voltages, and pulse widths for thermalink-jet printers, driving frequencies of the piezoelectric element foreither a drop-on-demand device or continuous device and the shape andsize of the nozzle.

The support for the lithographic printing plate is typically formed ofaluminum which has been grained for example by electrochemical grainingand then anodized for example by means of anodizing techniques employingsulfuric acid and/or phosphoric acid. Methods of both graining andanodizing are well known in the art.

After writing the image to the printing plate, the printing plate may beinked with printing inking the normal way and the plate used on aprinting press. Before inking the plate may be treated with an aqueoussolution of natural gum, such as gum acacia or of a synthetic gum suchas carboxymethylcellulose, as is known in the art of printing see forexample Chapter 10 of “The Lithographer's Manual” edited by CharlesShapiro and published by The Graphic Arts Technical Foundation, Inc.Pittsburgh, Pa. (1966).

Referring to the drawing: from an ink-jet printer head 2 droplets ofdispersion (solution or emulsion) 4 are jetted onto a hydrophilicsurface 6 of a printing plate 8. The direction of movement of theprinting head is indicated by the arrow. A hydrophobic image 10 isproduced on the support.

The invention is illustrated by the following Examples.

Preparation 1 Preparation of tetradecylthio′tri(acrylamidoglycolic acid)

The monomer (acrylamidoglycolic acid) (16.3 g, 100 mmol) was dissolvedin methanol (200 ml) and purged with nitrogen gas for 15 minutes. Themixture was heated to 62.5° C. and a solution of tetradecylmercaptan(7.68 g, 33.3 mmol) and AIBN (0.24 g) in methanol (50 ml) which had alsobeen purged with nitrogen gas, was added in one portion. The mixture wasrefluxed under an atmosphere of nitrogen gas for 3 hours. The cooledreaction mixture was washed with heptane (2×150 ml) and solvent removedby evaporation at reduced pressure to give 16.92 g of the water-solubleproduct with an average of 3 monomer units attached to the thiol.

EXAMPLE 1

A 1 wt % solution in water of the oligomeric surfactant prepared inPreparation 1 above was painted onto a piece of KODAK (Registered TradeMark) anodised aluminum printing plate using an artist's paintbrush tomake an image. The image was allowed to dry naturally and then the platewas wetted with a 0.05 wt % solution of “Viscofas”, a proprietarylithographic fountain preparation, using cotton wool. The plate was thenlightly rubbed with a small amount of printers ink (BASF FishburnsMinilith Black) on a piece of cotton wool. The image that had beenpainted on selectively took up the ink, showing a good representation ofthe painted image with minimal image degradation. The inked image wasresistant to firm rubbing.

EXAMPLE 2

In a similar manner a 1 wt % solution in water of the sodium salt ofPreparation 2 also showed selective inking of a painted image on a KODAK(Registered Trade Mark) anodised aluminum printing plate.

Preparation 2

Neutralisation of tetradecylthio′tri′(acrylamidoglycolic acid): thetrisodium salt of the oligomeric surfactant was prepared by dissolvingthe above product in water at a concentration of 1 wt % and titratingwith aqueous sodium hydroxide using electrochemical detection of theend-point.

EXAMPLE 3

In a similar manner a 1 wt % solution in water of the sodium salt ofPreparation 2 also showed selective inking of a painted image on a Kodak(Registered Trade Mark) anodised aluminum printing plate.

Preparation 3

Preparation ofoctadecylthio′tetra′(2-acrylamido-2-methyl-1-propanesulphonic acid):octadecanethiol (28.66 g, 0.10 mol ) and 2-acrylamido-2-methyl-1-propanesulfonic acid (82.9 g, 0.40 mol ) were stirred together withazobisisobutyronitrile (AIBN) (1.0 g) in methanol (500 ml). The mixturewas degassed with argon then refluxed under an argon atmosphere for 18hours. The reaction mixture still appeared a little cloudy. On cooling awhite solid began to form. The mixture was reheated on a steam bathwhich caused oily globules to appear. The hot solution was filteredunder suction on a sinter, trapping the oily globules as a white rubberysolid which failed to dissolve in water, sodium hydroxide or ethylacetate. The remaining solution on cooling gave another whitesemicrystalline solid which was filtered off. The remaining solution waswashed with heptane (500 ml) and the methanol evaporated to give theproduct as a white solid (92.4 g).

EXAMPLE 4

In a similar manner a 1 wt % solution in water of the oligomeric productfrom Preparation 3 also showed selective inking with good quality imageresistant to rubbing.

Preparation 4

Preparation ofoctadecathio′tetra′(2-acrylamido-2-methyl-1-propanesulphonic acid sodiumsalt). Based on titration results the product from Example 3 (50.00 g,0.0448 mol) was suspended in water (100 ml) in a round bottom flask andstirred at room temperature. Sodium hydroxide solution (10M, 18.60 ml)was added and the solution warmed to 45° C. for about 15 minutes thenallowed to cool while stirring continued (total time 0.5 hours). Theresulting pale yellow solution was freeze dried to give the product as awhite solid (50.1 g).

EXAMPLE 5

In a similar manner a 1 wt % solution of the oligomeric surfactant fromPreparation 4 showed similar behaviour.

Preparation 5

Preparation of Tyloxapol disulphate disodiuum salt. Tyloxapol is anovolak resin comprising an ethoxylated phenol formaldehyde resin.

Tyloxapol (5 g, about 1.3 mmol) was dissolved in 1,2-dichloroethane (100ml) and chlorosulphonic acid (0.3 g. 2.6 mmol) was added and the mixtureheated at 50° C. for 2.5 hours with exclusion of moisture. The mixturewas then cooled and solvent evaporated under reduced pressure. Water(100 ml) was and stirred to dissolve. The pH was adjusted to 10 to 11with aqueous sodium hydroxide and evaporated to dryness on a steam bath.The residue was treated with methanol and the inorganic salts filteredoff. The product was isolated by evaporation under reduced pressure anddried under high vacuum.

EXAMPLE 6

The oligomer of Preparation 2 was formulated according to the table togive 20 ml of solution which was placed in an empty clean ink-jetcartridge.

stock solutions vol used in component (wt %) ink (ml) oligomer 1 9.6ethanediol 15 1.4 sorbitol 5 1.0 water 8.0 total 20.0 

A standard test-object image was printed onto an Autotype Omega E-Zpolyester printing plate using an Epson 200 ink-jet printer, the imageallowed to dry and the plate then placed on a printing press (HeidelbergT-Offset) and run using Varn PressMaster Universal Fountain Solution(diluted 1 plus 15) and Van Son Rubber Based Ink-VS310 “Pantone” Blackto give clear prints of the test image after rapid ink-up.

EXAMPLE 7

A 0.5% weight aqueous solution of the product of preparation 5 wasprepared and the procedure of Example 2 repeated. Again selective inkingof the painted area was seen with good quality image.

What is claimed is:
 1. A printing plate comprising a hydrophilic supporthaving deposited thereon an image comprising an oligomer containing inthe molecule hydrophobic and hydrophilic groups wherein the latter serveto bind it to the support.
 2. A method for the preparation of alithographic printing plate which method comprises forming an oleophilicimage on the surface of a hydrophilic support by depositing the image onthe surface using an aqueous dispersion of an oligomer having in themolecule both hydrophilic and hydrophobic groups.
 3. A method as claimedin claim 2 wherein the oleophilic image is deposited on the surface byink-jetting.
 4. A method as claimed in claim 2 wherein the number ofhydrophilic groups in the oligomer is from 2 to
 10. 5. A method asclaimed in claim 2 wherein the molecular weight of the oligomer is from500 to
 5000. 6. A method as claimed in claim 2 wherein at least one ofthe hydrohilic groups is an acid group which has been neutralized.
 7. Amethod as claimed in claim 2 wherein the hydrophilic groups are selectedfrom carboxylic, sulphonic or phosphonic acids and the salts thereof.